This application is a continuation of a 371 of PCT/IL98/00422, filed on Aug. 31, 1998.
The present invention relates to the manufacture of electrical heating devices, particularly those employing thermistors with positive temperature coefficient of resistance (PTC) as heating elements.
Positive temperature coefficient (PTC) heating elements, such as thermistors, are used in electrical heating devices, such as electrical radiators, electrical heating fans, and air conditioner heaters. They have an advantage over electric wire heaters in that they are self-regulating as to temperature and thus are not subject to overheating even in response to abnormal electric currents. As with all heating devices, they must operate at elevated temperatures and must tolerate transitions between ambient and operating temperatures. As is known in the art, this thermal cycling introduces mechanical and other strains to such devices that can cause them to operate at reduced efficiency and can shorten their lifetime.
Prior art has attempted to address these problems by means of design of the heating elements. U.S. Pat. No. 4,954,692, which discloses a PTC thermistor heating device employing a type of spring to introduce pressure on the interface between the PTC thermistors and the radiators of the device, is typical of one approach. The thermal cycling that the device experiences in normal use still applies pressure variation on the PTC thermistors and their interface with the radiators which causes varying resistance, reduced efficiency, and shortened lifetime of the PTC elements and of the entire device. Another known approach is to introduce a thermally and possibly electrically conducting adhesive to the interface, as in U.S. Pat. No. 5,358,793. Devices employing adhesives fabricated according to known methods only partially alleviate the abovementioned problems and are subject to micro-fissuring in the adhesive layer and to interface breakdown when exposed to thermal cycling.
The present invention seeks to overcome the disadvantages of known art in positive temperature coefficient (PTC) thermistor electrical heating devices by providing an improved method for their fabrication. The method includes steps of pre-exposing the devices to operational temperatures while monitoring the electrical resistance of the devices and maintaining the stability thereof by adjusting the pressure across the interface between the thermistor heating elements and the radiators of the heating devices. Use of this method produces PTC thermistor electrical heating devices with an extended useful life compared to those produced according to known art.
The present invention further seeks to provide a system for fabricating positive temperature coefficient (PTC) thermistor electrical heating devices according to the abovementioned method.
There is thus provided, in accordance with a preferred embodiment of the invention, a method of fabricating positive temperature coefficient (PTC) thermistor electrical heating devices which includes: assembling the heating devices with PTC thermistor heating elements preselected according to electrical resistance, applying pressure across the interface between the PTC heating elements and the radiator plates of the heating devices during their assembly, and exposing the heating devices for extended periods of time to both ambient and operational temperatures. A further feature of the present method of fabrication is ensuring that any temperature changes are very gradual. The present method of fabrication further includes continuously monitoring the electrical resistance of the heating devices and holding their electrical resistance stable by adjusting the pressure applied across the interface between the PTC heating elements and the radiator plates.
There is further provided, in accordance with a preferred embodiment of the present invention, a system for performing the abovementioned fabrication method which includes a constant temperature chamber for controlling the temperature environment of the heating devices, an ohm-meter circuit for monitoring their electrical resistance, and an adjustable clamping device for adjusting the pressure across the interface between the thermistor heating elements and the radiators of the heating devices.